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The Hydrogeological Systems of Incoming Oceanic Plates and Overriding Convergent Margins of Subduction Zones: Insights from Studies of the Middle America Trench

The hydrogeological systems of incoming oceanic plates and overriding
convergent margins of subduction zones: Insights from studies of the
Middle America Trench.

César R. Ranero, ICREA at Instituto de Ciencias del Mar, CSIC, Pg. Marítim de la
Barceloneta 37-49, 08003 Barcelona, Spain. e-mail: cranero@icm.csic.es

At the WHOI geodynamics seminar I will present recent observations and conceptual
models of two important hydrogeological systems of subduction zones: The system of the
incoming oceanic plate and the forearc system. I will also present some ideas on the
implications of the models for shallow and deep tectonic processes and recycling of water
in the mantle.

The physical and chemical structure of subducting oceanic slabs is presently poorly
known. After the realization of the meaning of oceanic trenches in the late 60’s, most
research in the 70’s and early 80’s was dedicated to explain their large-scale geometry.
Subsequent research on subduction zones has been greatly focused on overriding plates,
and the oceanic plates have been comparatively overlooked. However, recent highresolution
data indicate that oceanic lithosphere physical and chemical structure changes
dramatically just prior to subduction. Those changes which -with the limited available
data- we are just starting to appreciate, possibly have major implications for a number of
first order processes, like the storage and transport of water into the subarc mantle (and
perhaps deeper) and the generation of intermediate depth (~70-350 km depth) seismicity.
When oceanic plates bend to form the trench (prior to subduction) faulting -related to
bending- cuts into the lithosphere across the igneous crust and into the upper mantle. The
data indicate that faults possibly provide open paths for water percolation deep into the
plates, where fluids may alter the oceanic crust and transform mantle peridotites into
serpentine. The hydration of oceanic lithosphere at trenches may provide most of the fluids
transported in slabs and released under arcs, and perhaps some fluids that are transported
deeper into the mantle. Plate hydration may also play an important role on the occurrence
of intra-slab intermediate-depth earthquakes.

The distribution and flow of fluid is commonly related to tectonics at all settings. At
convergent margins this relationship has been widely studied at accretionary prisms, but at
convergent margins where tectonic erosion affects overriding plates fluid distribution and
tectonics are far less understood.

An integrated study of geophysical, geochemical and geological observations along the
erosional type Middle America Trench indicates a hydrogeological system distinctly
different from those described at accretionary prisms. The observations show how the
hydrogeological system influences long-term tectonic erosion and the transition with depth
from aseismic to seismogenic behavior along the plate boundary, where large earthquakes
nucleate. In this hydrogeological system most fluid appears to come from pore water or
chemically-bond water in subducting sediment. Where fluid is more abundant along the
plate boundary, the overriding plate is actively being thinned, and fractures and subsides to
form the continental slope. Most fluid originally contained at the plate boundary migrates
by focused flow across a fractured overriding plate, contrasting with conceptual models of
accretionary margins where the decollement has been inferred to be the main fluid flow
conduit. Seismogenic behavior at the plate boundary begins where fluid appears to be less
abundant indicating a first order control on subduction zone thrust earthquakes.

Originally published: February 21, 2008